This invention relates to a switching type AC adapter circuit and, in particular, to a switching type AC adapter circuit with a latch protection circuit.
A switching type AC adapter circuit of the type described comprises a primary side circuit for turning, by using a switching element, a DC voltage applied to a primary winding of a transformer on and off and a secondary side circuit for rectifying and smoothing a current induced in a secondary winding of the transformer to produce a secondary side output voltage.
In such a switching type AC adapter circuit, it is necessary to electrically isolate and separate the primary side circuit from the secondary side circuit in order to prevent an accident such as an electric shock. As means for electrically isolating and separating, a photocoupler or an isolation transformer is generally used. In the switching type AC adapter circuit, it is necessary to feed a variation of the secondary side output voltage as a constant voltage control signal back to the primary side circuit in order to carry out a constant voltage control. In this event, the constant voltage control signal is feed from the secondary side circuit back to the primary side circuit through a constant voltage controlling photocoupler.
The switching type AC adapter circuit may detect an overcurrent or an overvoltage in the secondary side circuit to feed an overcurrent detected signal or an overvoltage detected signal back to the primary side circuit. In this event, the primary side circuit comprises a latch circuit for making a switching operation of the switching type AC adapter circuit stop when the overcurrent or the overvoltage is detected in the secondary side circuit. In addition, the above-mentioned overcurrent detected signal of the above-mentioned overvoltage detected signal is transferred or fed from the secondary side circuit back to the latch circuit of the primary side circuit through a latching photocoupler.
In the manner which will later be described in conjunction with FIGS. 1–3, a conventional switching AC adapter circuit comprises not only a constant voltage controlling photocoupler but also a latching photocoupler. Accordingly, it is desired that one photocoupler used for a normal control (e.g. a constant voltage control or the like) transmits also an abnormal state (e.g. overvoltage, overcurrent, or the like).
In connection with this, Japanese Patent Publication No. 2000-156,972 or JP-A 2000-156972 discloses a switching power supply apparatus for overlapping, by using one switch, an overvoltage protection signal (or a remote control signal) to a constant voltage detection control signal for a main output voltage induced in a secondary side circuit to transmit an overlapped signal to a primary side circuit by isolating the primary side circuit with the secondary side circuit using one photocoupler.
However, the switching power supply apparatus disclosed in JP-A 2000-156972 is disadvantageous in that a primary control circuit disposed in the primary side circuit is complicated in structure and a different auxiliary power supply portion is required.